Environmental Engineering Reference
In-Depth Information
34
S,
Table 3.
Sulfate reduction rate (SRR) and the isotopic composition of hydrogen sulfide (δ
‰) in the deep part of the Black Sea water column (unpublished data).
SRR, nmol 1
−
1
d
−
1
34
S,
‰
Station, water depth
Sampling depth, m
δ
Station 4988, 1989 m
180
8.4
-40.3
190
-
-40.3
200
43.1
-
210
-
-41.4
250
74.0
-41.3
275
-
-41.6
300
9.0
-
350
10.3
-41.7
Station 839, 2160 m
500
4.0
-40.0
700
4.0
-
1500
-
-40.2
2100
35
-40.4
34
S at the boundary between reduced and oxidized
zones of the Black Sea compared to deeper layers (Tables 2-4) are supported by
Muramoto's data [78]. The material collected from sedimentary traps at water
depths of 500 and 1000 m contained pyrite produced at the upper boundary of
H
2
S zone with δ
sulfide is less depleted in δ
34
S values from -32.7 to -38.5‰ [78].
3. FRACTIONATION OF SULFUR ISOTOPES
DURING SULFATE REDUCTION IN THE UPPER
HOLOCENE SEDIMENTS OF THE BLACK SEA
In the Upper Holocene sediments of the Black Sea, anaerobic decomposi-
tion of organic matter by sulfate-reducing bacteria is the main mineralization
process. In sediments of the anaerobic zone (at water depths below 200 m), the
most active process of sulfate reduction was detected in the upper sediments
(in the so-called 'fluff layer') rich in organic matter [1, 46, 62, 66, 91, 97].
Even in shallow-water sediments, sulfate reduction occurs in the uppermost
sediments covered with a thin (less than 1 mm) layer of brown silt [65, 69, 70,
73, 97]. Only in wintertime when water temperature drops and the influx of
allochthonous and autochtonous organic matter decreases, the uppermost shelf
sediments become aerobic. Active sulfate reduction leads to the accumulation
of reduced sulfur compounds in sediments - up to 1.5% d.w.[69, 95]. It is
followed by the fractionation of the stable sulfur and carbon isotopes. Table 5
shows fluctuations of δ
34
S of terminal products of the reduction processes in
more than a hundred samples of sedimentary pyrite and pore water sulfates.
